Solids-free, essentially all-oil wellbore fluid

ABSTRACT

A solids-free, essentially all-oil wellbore fluid comprises an organic fluid having (i) a melting point less than about 20° C. (about 68° F.), (ii) a flash point greater than about 54.4° C. (about 130° F.), and (iii) a dipole moment greater than 0 debye (D) and/or an aromatic solvent having a density at about 15.6° C. (60° F.) of at least about 0.9 g/ml (7.5 pounds per gallon (ppg)), a flash point of greater than about 54.4° C. (130° F.), a solubility in water at about 25° C. (77° F.) of less than about 1 weight percent, a solubility in benzene at about 25° C. (77° F.) of at least about 80 weight percent, and a pour point of less than about 15.6° C. (60° F.). Generally, a salt is dissolved in the organic fluid. The wellbore fluid is employed in well drilling, completion, and work-over operations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/055,510,filed Apr. 30, 1993, now abandoned which application is acontinuation-in-part of application Ser. No. 07/948,509, filed Sep. 21,1992, now U.S. Pat. No. 5,556,832 which continuation-in-part applicationis incorporated herein in its entirety by reference.

BACKGROUND

The present invention relates to (a) solids-free, non-aqueous wellborefluids of variable high density, (b) methods for using such wellborefluids during or after drilling to (i) complete and/or treat aproduction or injection well or (ii) treat and/or modify a subterraneanformation, and (c) natural resource systems containing such wellborefluids. (As used in the specification and claims, the term "wellborefluid" means a fluid used while conducting pay zone drilling,underreaming, drilling in, plugging back, sand control, perforating,gravel packing, chemical treatment, hydraulic fracturing, cleanout, wellkilling, tubing and hardware replacement, and zone selective operationsas well as a fluid employed as a packer fluid. The term "solids-free" isapplied to the basic wellbore fluid having the desired specific gravity.As understood in the art, the term "solids-free" means that no solidmaterial (e.g., weighting agents, viscosifiers, fluid loss controladditives) is present in the wellbore fluid. Nevertheless, in certaincases, solid additives can be added to the wellbore fluid for specificpurposes.)

Aqueous base completion fluids can cause swelling of clay-containingstructures in a pay zone. For example, reservoir rocks containingvolcanic ash and/or smectic or mixed layer clays could be permanentlydamaged if contacted with an aqueous base fluid. In addition,brine-in-oil emulsions can also cause clay swelling due to the internalwater phase of the emulsion. Furthermore, the emulsifiers present inbrine-in-oil emulsions can cause detrimental formation wettabilitychanges.

Clean hydrocarbon oils (e.g., crude oil) are the least damagingcompletion fluids to be placed across an oil-bearing formation. See, forexample, European Patent Application No. 87304548.8 and SPE 17161. Untilrecently, there was no oil-soluble material available to increase thedensity of the oil. See SPE 17161. However, European Patent ApplicationNo. 87304548.8 discloses that halogenated organic compounds (e.g.,brominated aromatic ethers, diphenyls, aliphatic hydrocarbons, benzene,and alkyl benzenes) can be dissolved in an organic solvent such as crudeoil, kerosene, diesel oil or a low toxicity drilling oil. Unfortunately,these halogenated hydrocarbons have several drawbacks. For example, theytend to be very costly, can be environmentally hazardous, and may causeadverse effects on downstream processing equipment (e.g., catalystpoisoning).

SUMMARY OF THE INVENTION

It would be desirable to have solids-free, non-aqueous completion orwellbore fluids that do not possess the disadvantages of the halogenatedorganic compounds of European Patent Application No. 87304548.8.Furthermore, it would be desirable to have a method for furtherincreasing the density of the halogenated organic compounds disclosed inEuropean Patent Application No. 87304548.8 to reduce the amount of suchhalogenated organic compounds (e.g., by further dilution with ahydrocarbon diluent) required in a well completion or work-overprocedure.

The present invention provides (A) additional solids-free wellborefluids, (B) drilling, well completion, and work-over methods employingsuch fluids, and (C) natural resource systems containing such fluids. Inone embodiment of the present invention, the solids-free wellbore fluidcomprises an aromatic solvent having a density at about 15.6° C. (60°F.) of at least about 0.9 g/ml (7.5 pounds per gallon (ppg)), a flashpoint of greater than about 54.4° C. (130° F.), a solubility in water atabout 25° C. (77° F.) of less than about 1 weight percent, a solubilityin benzene at about 25° C. (77° F.) of at least about 80 weight percent,a viscosity at about 37.8° C. (100° F.) of less than about 0.2 newtonsecond/meter² (200 cps), and a pour point of less than about 15.6° C.(60° F.).

In another version of the invention, the wellbore fluid comprises anorganic fluid. (As used in the specification and claims, the term"organic fluid" means a carbon-containing compound having (i) a meltingpoint less than about 20° C. (about 68° F.), (ii) a flash point greaterthan about 54.4° C. (about 130° F.), and (iii) a dipole moment greaterthan 0 debye (D).) The organic fluid comprises one or more halogenatedcompounds (such as those described in European Patent Application No.87304548.8) and/or one or more unhalogenated compounds. When a lowdensity organic fluid is employed (e.g., an organic fluid having adensity of less than about 1 g/ml (8.35 pounds per gallon (ppg)), a saltis generally dissolved in the organic fluid to increase the density ofthe organic fluid while enabling the organic fluid to remainsolids-free. When a high density organic fluid is used (e.g., an organicfluid having a density of at least about 1 g/ml (8.35 ppg)), the organicfluid is commonly dissolved in a hydrocarbon diluent (e.g., the aromaticsolvent) to increase the density of the hydrocarbon diluent.

Optionally, the wellbore fluids of the present invention furthercomprise the hydrocarbon diluent (e.g., crude oil, kerosene, diesel oil,polyalphaolefins (such as those described in U.S. Pat. No. 5,096,883,which patent is incorporated herein in its entirety by reference),mineral oil, gasoline, naphtha, aromatic solvents, and mixtures thereof)and/or an additive (e.g., acids, bases, buffers, viscosifiers, corrosioninhibitors, antioxidants, proppants for use in hydraulically fracturingsubterranean formations, particulate agents for use in forming a gravelpack, organophilic clays, fluid loss control agents, and mixturesthereof).

The wellbore fluids of the present invention can be employed invirtually any well drilling or completion or work-over operation (e.g.,pay zone drilling, underreaming, drilling in, plugging back, sandcontrol, perforating, gravel packing, chemical treatment, hydraulicfracturing, cleanout, well killing, tubing and hardware replacement, andzone selective operations). In addition, the wellbore fluids can be usedas a packer fluid.

Regarding the natural resource system of the present invention, suchsystem comprises a subterranean formation (generally having present inat least a portion thereof a natural resource such as crude oil, naturalgas, and/or a geothermal fluid), a well penetrating at least a portionof the subterranean formation, and the solids-free wellbore fluidpresent, for example, in at least a portion of the well and/or thesubterranean formation.

DETAILED DESCRIPTION OF THE INVENTION

The organic fluid employed in the present invention preferably has amelting point less than about 16° C. (about 60° F.), more preferablyless than about 10° C. (about 50° F.), even more preferably less thanabout 5° C. (about 41° F.), and most preferably less than about 0° C.(about 32° F.). The flash point of the organic fluid is preferablygreater than about 60° C. (about 140° F.), more preferably greater thanabout 65.6° C. (about 150° F.), and most preferably greater than about71.1° C. (about 160° F.). In the embodiments of the invention where asalt is dissolved in the organic fluid, the dipole moment of the organicfluid is preferably greater than about 0.5, more preferably greater thanabout 1, and most preferably greater than about 1.5 D. In general, whenall other factors (e.g., cost, toxicity, melting and flash points) arethe same and when a salt is dissolved in the organic solvent, it ispreferred to employ the organic fluid having the highest dipole moment.When a salt is not dissolved in the organic fluid, the organic solventneed not have a high dipole moment and commonly has a density of atleast about 1 g/ml (8.35 ppg), preferably at least about 1.05 g/ml (8.77ppg), more preferably at least about 1.1 g/ml (9.19 ppg), even morepreferably at least about 1.15 g/ml (9.60 ppg), and most preferably atleast about 1.2 g/ml (10.02 ppg).

Generally, the organic fluid employed in the present invention has asolubility in 100 g of water at 25° C. (77° F.) of less than about 10,preferably less than about 5, more preferably less than about 1, andmost preferably less than about 0.1 g. In fact, it is even desirable forthe organic fluid to be substantially insoluble and even more desirablefor the organic fluid to be virtually insoluble in 100 g water at 25° C.(77° F.). (As used in the specification and claims, the term"substantially insoluble" when used in conjunction with the solubilityof the organic fluid in water means that less than about 0.01 g of theorganic fluid is soluble in 100 g water at 25° C. (77° F.); and the term"virtually insoluble" when used in conjunction with the solubility ofthe organic fluid in water means that less than about 0.001 g of theorganic fluid is soluble in 100 g water at 25° C. (77° F.).)

Exemplary classes of organic fluids for use in the present inventioninclude, but are not limited to, aryl halides (usually containing about6 to about 7 carbon atoms), heterocyclic compounds (generally containingabout 5 to about 9 carbon atoms), alkyl halides (typically containingabout 6 to about 8 carbon atoms), carboxylic acids (commonly containingabout 4 to about 18 carbon atoms), amines (often containing about 6 toabout 16 carbon atoms), esters (frequently containing about 6 to about16 carbon atoms), alcohols (ordinarily containing about 6 to about 16carbon atoms), aldehydes (commonly containing about 7 to about 8 carbonatoms), ketones (generally containing about 6 to about 12 carbon atoms),ethers (usually containing about 8 to about 14 carbon atoms), plantoils, and animal oils. The organic fluids are employed in the inventionindividually or in any combination thereof. Representative organicfluids are set forth in the following Table I:

                  TABLE I                                                         ______________________________________                                        Representative Organic Fluids                                                 Class        Species                                                          ______________________________________                                        Aryl Halides halotoluene.sup.1, dihalotoluene,                                             dihalobenzene, dihaloalkylbenzene.sup.2                          Heterocyclic furfural, quinoline                                              Compounds                                                                     Alkyl Halides                                                                              octyl halide.sup.1, cyclohexyl halide                            Carboxylic   valeric acid, caproic acid, heptanoic                            Acids        acid, octanoic acid, nonanoic acid, oleic                                     acid, linoleic acid, linolenic acid, 2-                                       methyl propionic acid, 3-methyl butanoic                                      acid                                                             Amines       aniline, methyl aniline, dimethyl aniline,                                    toluidine, anisidine, haloaniline.sup.1,                                      tripropylamine, triamyl amine, heptyl                                         amine, dicylcohexyl amine, dibutylamine,                                      tributyl amine, monobutyl diamylamine,                                        octylamine, dioctylamine                                         Esters       2-ethoxyethyl acetate, ethylene glycol                                        diacetate, 2-butoxyethyl acetate, 2-                                          ethylhexyl acetate, 2-(2-                                                     ethoxyethoxy)ethyl acetate, 2-(2-                                             butoxyethoxy)ethyl acetate, glyceryl                                          triacetate, 2,2,4-trimethyl pentanediol,                                      diisobutyrate, glyceryl tributyrate,                                          tributyl phosphate, dimethyl phthalate,                                       diethyl phthalate, dipropyl phthalate,                                        dibutyl phthalate, benzyl acetate, bis(2-                                     ethylhexyl) adipate, undecanoic γ-lactone                  Alcohols     hexanol, heptanol, octanol, nonanol,                                          decanol, ethylhexanol, octanol, isoctyl                                       alcohol, cyclohexanol, isodecanol, benzyl                                     alcohol, phenylethanol, 3,5-dimethyl-1-                                       hexanol, 2,2,4-trimethyl-1-pentanol, 2,6-                                     dimethyl-4-heptanol, 3,3,5-                                                   trimethylhexanol, diacetone alcohol,                                          furfuryl alcohol, 2-heptyl alcohol                               Aldehydes    heptaldehyde, octanal, benzaldehyde,                                          tolualdehyde, phenylacetaldehyde,                                             salicylaldehyde, anisaldehyde,                                                tetrahydrobenzaldehyde                                           Ketones      2,5-hexanedione, 2,6,8-trimethyl                                              isobutylheptylketone, butyrophenone,                                          methyl heptyl ketone, cyclohexanone                              Ethers       phenetole, hexyl ether, dibenzyl ether,                                       butylphenyl ether, amyl phenyl ether, amyl                                    benzyl ether, amyl tolyl ether, octyl                                         phenyl ether, hexyl phenyl ether                                 Plant Oils   pine oil, linseed oil, canola oil, soybean                                    oil, corn oil, peanut oil, rapeseed oil,                                      sunflower oil, palm oil, olive oil                               Animal Oils  Animal fats                                                      ______________________________________                                         .sup.1. Exemplary halides are bromine, chloride, and iodine.                  .sup.2. The alkyl qroup generally contains 1 to about 6 carbon atoms with     about 2 carbon atoms being preferred.                                    

The preferred organic fluids are esters and alcohols.

The salts dissolved in the organic fluid are generally inorganic salts.Exemplary inorganic salts include, but are not limited to, zinc halides,alkaline earth metal halides, cadmium halides, alkali metal halides, tinhalides, arsenic halides, copper halides, aluminum halides, silvernitrate, mercury halides, mercuric cyanide, lead nitrate, coppersulfate, nickel halides, cobalt halides, manganese halides, and chromiumhalides. The preferred halides are chlorine, bromine, and iodine; thepreferred alkali metals are lithium, sodium, potassium, rubidium, andcesium; and the preferred alkaline earth metals are magnesium, calcium,strontium, and barium. An individual salt as well as combinations of twoor more salts are used in the wellbore fluid.

The concentration of the salt in the organic fluid depends on thedesired density of the wellbore fluid. In general, any concentration ofsalt up to the solubility limit of the salt in the organic fluid can beused. Typically, the wellbore fluid contains at least about 0.1,preferably at least about 1, more preferably at least about 10, evenmore preferably at least about 25, and most preferably at least about50, weight percent dissolved salt. (As used in the specification andclaims, the term "weight percent" when used to designate theconcentration of the dissolved salt in the wellbore fluid means theweight of the dissolved salt in the wellbore fluid divided by the sum ofthe weights of the organic fluid and dissolved salt in the wellborefluid, the quotient being multiplied by 100 percent.) Quite often, thesolubility limit of the salt in the wellbore fluid is less than about 75weight percent, more typically less than about 50 weight percent, andusually less than about 25 weight percent.

In another embodiment of the present invention, the wellbore fluidcomprises an aromatic solvent. In this version of the invention, thearomatic solvent generally has a density at about 15.6° C. (60° F.) ofat least about 0.9 g/ml (7.5 ppg), preferably at least about 0.925 g/ml(7.72 ppg), more preferably at least about 0.95 g/ml (7.93 ppg), evenmore preferably at least about 0.975 g/ml (8.14 ppg), and mostpreferably at least about 1 g/ml (8.35 ppg). Typically, the aromaticsolvent has a flash point greater than about 54.4° F. (about 130° F.),preferably greater than about 60° C. (about 140° F.), more preferablygreater than about 65.6° C. (about 150° F.), and most preferably greaterthan about 71.1° C. (about 160° F.). The pour point of the aromaticsolvent is usually less than about 15.6° C. (60° F.), preferably lessthan about 4.4° C. (40° F.), and more preferably less than about -6.7°C. (20° F.). Commonly, the aromatic solvent has a viscosity of less thanabout 0.2 newton second/meter² (N-sec/m²) (200 cps), with the viscositybeing preferably less than about 0.15 N-sec/m2 (150 cps), morepreferably less than about 0.1 N-sec/m2 (100 cps), even more preferablyless than about 0.05 N-sec/m2 (50 cps), and most preferably less thanabout 0.025 N-sec/m² (25 cps).

Regarding the solubility of the aromatic solvent in water and benzene,the solubility of the aromatic solvent in water at 25° C. (77° F.) isgenerally less than about 1, preferably less than about 0.5, morepreferably less than about 0.25, and most preferably less than about 0.1weight percent. In fact, it is even desirable for the aromatic solventto be substantially insoluble and even more desirable for the organicfluid to be virtually insoluble in 100 g water at 25° C. (77° F.). (Asused in the specification and claims, the term "substantially insoluble"when used in conjunction with the solubility of the aromatic solvent inwater means that the solubility of the aromatic solvent in water at 25°C. (77° F.) is less than about 0.01 weight percent; and the term"virtually insoluble" when used in conjunction with the solubility ofthe aromatic solvent in water means that the solubility of the aromaticsolvent in water at 25° C. (77° F.) is less than about 0.001 weightpercent.)

In benzene at 25° C. (77° F.), the aromatic solvent has a solubility ofgenerally at least about 80, preferably at least about 85, morepreferably at least about 90, even more preferably at least about 95,and most preferably at least about 99 weight percent. In fact, it ispreferred that the aromatic be completely miscible in benzene at 25° C.(77° F.).

Exemplary aromatic solvents meeting the above requirements are set forthin Mellan, Handbook of Solvents, Volume 1, Reinhold PublishingCorporation, New York, N.Y. (1957) and Mardsen, Solvents Guide, SecondEdition, Interscience Publishers, A Division of John Wiley and Sons,Inc., New York, N.Y. (1963), both of these publications beingincorporated in their entireties by reference. Preferred aromaticsolvents include those listed in the following Table II:

                  TABLE II                                                        ______________________________________                                        Aromatic Solvents                                                             Name        Description                                                       ______________________________________                                        PANASOL AN-3S.sup.1                                                                       Density of about 0.992 g/ml (8.28                                             ppg); boiling point range of about 210° to                             about 287.8° C. (410°-550° F.); flash                    point of                                                                      about 87.8° C. (190° F.); an aromatic content                   of about 99%; containing substituted mono-                                    and di-alkylnapthalenes.                                          PANASOL AN-3N.sup.1                                                                       Density of about 0.995 g/ml (8.31                                             ppg); boiling point range of about 232.2°                              to about 287.8° C. (450°-550° F.); flash                 point                                                                         of about 85° C. (185° F.); an aromatic content                  of about 99%; containing substituted mono-                                    and di-alkylnapthalenes.                                          SOLVENT H-T.sup.2                                                                         Density of about 0.994 g/ml (8.3                                              ppg); boiling point range of about 226.7°                              to about 390° C. (440°-734° F.); flash                   point                                                                         of about 101.7° C. (215° F.); an aromatic                       content of about 75%.                                             AROMATIC    Density of about 0.958 g/ml (8.0                                  SOLVENT 400.sup.3                                                                         ppg); boiling point range of about 207.2°                              to about 346.1° C. (405°-655° F.).           ______________________________________                                         .sup.1. Available from Amoco.                                                 .sup.2. Available from AMSCO.                                                 .sup.3. Available from Texaco.                                           

The wellbore fluids of the above invention embodiments optionallycontain one or more additional ingredients such as hydrocarbon diluents,proppants suitable for use in hydraulically fracturing subterraneanformations, particulate agents suitable for use in forming a gravelpack, corrosion inhibitors, acids, bases, buffers, viscosifiers,antioxidants, organophilic clays, and fluid loss control agents. Typicalhydrocarbon diluents include, but are not limited to, crude oil,kerosene, diesel oil, polyalphaolefins, mineral oil, gasoline, naphtha,and aromatic solvents. In a preferred embodiment of the presentinvention, a high density organic fluid (e.g., an ester such as adialkyl phthalate having 1 to about 4 carbon atoms) is dissolved in thehydrocarbon diluent.

The concentration of the hydrocarbon diluent in the wellbore fluiddepends on the desired density of the wellbore fluid. Since thehydrocarbon diluent usually costs less than the organic fluid, it isusually desirable to use as much hydrocarbon diluent in the wellborefluid as possible while achieving the desired density of the wellborefluid. Generally, the hydrocarbon diluent is present in the wellborefluid in a concentration of at least about 5, preferably at least about10, more preferably at least about 25, even more preferably at leastabout 50 weight percent, and most preferably at least about 70 weightpercent. (As used in the specification and claims, the term "weightpercent" when used to designate the concentration of the hydrocarbondiluent in the wellbore fluid means the weight of hydrocarbon diluent inthe wellbore fluid divided by the sum of the weights of the organicfluid, the dissolved salt, and the hydrocarbon diluent in the wellborefluid, the quotient being multiplied by 100 percent.) While higherhydrocarbon diluent concentrations can be employed in the wellborefluid, the wellbore fluid usually contains about 95 weight percent orless, commonly about 90 weight percent or less, more commonly about 85weight percent or less, even more commonly about 80 weight percent orless, and most typically about 75 weight percent or less, hydrocarbondiluent.

When the hydrocarbon diluent is employed in conjunction with an organicfluid containing a dissolved salt, the dissolved salt-containing organicfluid is preferably miscible in the hydrocarbon diluent in at least theamount added to the hydrocarbon diluent. More preferably, the dissolvedsalt-containing organic fluid is miscible in the hydrocarbon diluent inall concentrations.

Common proppants suitable for use in hydraulic fracturing procedures arequartz sand grains, tempered glass beads, sintered bauxite, resin coatedsand, aluminum pellets, and nylon pellets. Generally, the proppants areemployed in the wellbore fluids of the present invention intended foruse as hydraulic fracturing fluids and are used in concentrations ofroughly about 1 to about 10 pounds per gallon of the wellbore fluid. Theproppant size is typically smaller than about 2 mesh on the U.S. SieveSeries scale, with the exact size selected being dependent on theparticular type of formation to be fractured, the available pressure andpumping rates, as well as other factors known to those skilled in theart.

Typical particulate agents employed in the wellbore fluids of thepresent invention used as gravel packing fluids include, but are notlimited to, quartz sand grains, glass beads, synthetic resins, resincoated sand, walnut shells, and nylon pellets. The gravel packparticulate agents are generally used in concentrations of about 1 toabout 20 pounds per gallon of the wellbore fluid. The size of theparticulate agent employed depends on the type of subterraneanformation, the average size of formation particles, and other parametersknown to those skilled in the art. Generally, particulate agents ofabout 8 to about 70 mesh on the U.S. Sieve Series scale are used.

Some of the organic fluids (e.g., aniline) which can be employed in thepresent invention also function as corrosion inhibitors. When such dualacting organic fluids are used in the present invention, there isgenerally no need for an additional corrosion inhibitor. When such dualacting organic fluids are not employed in formulating the wellbore fluidor when an additional corrosion inhibitor is desired, the corrosioninhibitor selected can be an inorganic and/or organic compound.

Inorganic corrosion inhibitors include, but are not limited to,chromates (e.g., sodium chromate), phosphates (e.g., sodium phosphate),nitrites, silicates, borates, and arsenic. When used, the inorganiccorrosion inhibitors are preferably present in the wellbore fluid in aconcentration of at least about 0.0001, more preferably at least about0.0005, and most preferably at least about 0.001, moles per liter of thewellbore fluid. The maximum concentration of the inorganic corrosioninhibitors in the wellbore fluid is generally less than about 0.1,preferably less than about 0.05, and more preferably less than about0.01, moles per liter of the wellbore fluid.

Exemplary organic compounds capable of functioning as a corrosioninhibitor in the wellbore fluid of the present invention include, butare not limited to, pyridine, butylamine, benzoic acid, benzosulfonicacid, nonamethyleneamine, diphenyl urea, carbon disulfide,allylthiourea, octyldecylamine, and hexadecylamine. When employed in thewellbore fluid, the organic corrosion inhibitors are preferably presentin a concentration of at least about 0.1, more preferably at least about0.5, and most preferably at least about 1, weight percent based on theentire weight of the wellbore fluid. Typically, the maximumconcentration of the organic corrosion inhibitor in the wellbore fluidis less than about 10, preferably less than about 5, and most preferablyless than about 2.5, weight percent based on the entire weight of thewellbore fluid.

Acids, bases, and buffers are employed in the wellbore fluid to helpmaintain the dissolved salts in solution when the wellbore fluid iscontacted by subterranean materials (e.g., water) having a pH capable ofcausing the precipitation of the dissolved salts. Some of the organicfluids employed in the present invention are acids (e.g., carboxylicacids) or bases (e.g., aniline, octylamine, quinoline) and, when used,generally negate the need for any additional acid or base, respectively.When acidic organic fluids are not used or when it is desired to use anadditional acidic component in the wellbore fluid, the acid selected canbe one or more inorganic and/or organic compounds. Common inorganicacids are hydrochloric acid, hydrobromic acid, hydrofluoric acid, nitricacid, phosphoric acid, orthophosphoric acid, sulfurous acid, sulfuricacid, boric acid, carbonic acid, chromic acid, hydroiodic acid,percholic acid, and alumic acid. Typical organic acids include oxalicacid, formic acid, caprylic acid, oleic acid, ascorbic acid, benzoicacid, butyric acid, lactic acid, acetic acid, and citric acid.

When basic organic fluids are not used or when it is desired to use anadditional basic component in the wellbore fluid, the base selected canbe one or more inorganic and/or organic compounds. Illustrativeinorganic bases are hydroxides (e.g., ammonium, alkali, and alkalineearth metal hydroxides), bicarbonates (e.g., alkali bicarbonate),carbonates (e.g., alkali carbonates), lime, and ammonia. Exemplaryorganic bases are acetamide, ethylenediamine, hydrazine, pyridine,benzylamine, butylamine, thiazole, toluidine, and urea.

The buffering agents employed in the present invention generally have abuffering capacity in a least a portion of the pH range of about 6 toabout 8, preferably about 6.5 to about 7.5, and most preferably about6.8 to about 7.2. Buffer agents having a buffering capacity in at leasta portion of the above pH ranges are set forth in Lange's Handbook ofChemistry, Editor: John A. Dean, 12th Edition, McGraw-Hill Book Co., NewYork, N.Y. (1979), pages 5-73 to 5-84, this publication beingincorporated herein in its entirety by reference. More specifically,phosphates (e.g., potassium dihydrogen phosphate, disodium monohydrogenphosphate), phosphate-hydroxide combinations (e.g., potassium dihydrogenphosphate and sodium hydroxide), phosphate combinations (e.g., potassiumdihydrogen phosphate and disodium monohydrogen phosphate),2-(N-morpholino)ethanesulfonic acid-sodium hydroxide combinations,2,2-bis(hydroxymethyl)-2,2',2"-nitriloethanol-hydrochloric acidcombinations, potassium dihydrogen phosphate-borax combinations,N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid-sodium hydroxidecombinations, triethanolamine-hydrochloric acid combinations, anddiethylbarbiturate-hydrochloric acid combinations are some of thebuffering agents having a buffering capacity within at least a portionof the aforementioned pH ranges.

The concentration of acid, base, or buffer employed in the wellborefluid is dependent upon the subterranean conditions that the wellborefluid is expected to encounter (e.g., the amount and pH of subterraneanwater expected to be in contact with the wellbore fluid). In general,when employed, the acid, base, or buffer is used in a concentration ofat least about 0.01, preferably at least about 0.05, and more preferablyat least about 0.1 weight percent based on the entire weight of thewellbore fluid. Typically, the maximum concentration of the acid, base,or buffer in the wellbore fluid is less than about 10, preferably lessthan about 5, and more preferably less than about 1 weight percent basedon the entire weight of the wellbore fluid.

Exemplary antioxidants employed in the present invention are2,6-ditertbutyl-p-cresol, butylated-hydroxy-anisole (BHA),butylated-hydroxy-toluene (BHT), tert-butyl-hydroquinone (TBHQ),o-cyclohexylphenol, and p-phenylphenol. When used, the antioxidants aregenerally present in the wellbore fluid in a concentration of at leastabout 0.0015, preferably at least about 0.01, and more preferably atleast about 0.1, but typically less than about 10, preferably less thanabout 5, and more preferably less than about 1, weight percent based onthe entire weight of the wellbore fluid.

Illustrative viscosifiers, organophilic clays, and fluid loss control(FLC) agents optionally used in the present invention as well as theirgeneral and preferred concentrations in the wellbore fluid are set forthin the following Table III.

                  TABLE III                                                       ______________________________________                                        Exemplary Viscosifiers, Clays, And FLC Agents                                                      Concentration, v %.sup.1                                 Ingredient                                                                            Species            General  Preferred                                 ______________________________________                                        Viscosifier                                                                           ethylene-propylene-diene monomer                                                                 0.02-2   0.05-1.5                                          (EPDM) terpolymers, copolymers                                                of isoprene and styrene sulfonate                                             salt, copolymers of chloroprene                                               and styrene sulfonate salt,                                                   copolymers of isoprene and                                                    butadiene, copolymers of styrene                                              and styrene sulfonate salt,                                                   copolymers of butadiene and                                                   styrene sulfonate salt, copolymers                                            of butadiene and styrene, ter-                                                polymers of isoprene, styrene, and                                            styrene sulfonate salt, terpolymers                                           of butadiene, styrene, and styrene                                            sulfonate salt, butyl rubber,                                                 partially hydrogenated polyiso-                                               prenes, partially hydrogenated                                                polybutylene, partially hydrogen-                                             ated natural rubber, partially                                                hydrogenated buna rubber,                                                     partially hydrogenated polybuta-                                              dienes, Neoprene, polymeric fatty                                             acids, hydroxylamine-esters, and                                              aluminates                                                            Organophilic                                                                          amine-treated bentonite, hectorite,                                                               0.5-10  1-5                                       Clay    illite, and attapulgite                                               FLC Agent                                                                             asphaltics (e.g., asphaltenes and                                                                   1-10  2-5                                               sulfonated asphaltenes), amine-                                               treated lignite, amine-treated                                                gilsonite, polystyrene, polybuta-                                             diene, polyethylene, polypropyl-                                              ene, polybutylene, polyisoprene,                                              natural rubber, butyl rubber, poly-                                           mers consisting of at least two                                               monomers selected from the group                                              consisting of styrene, butadiene,                                             isoprene, and vinyl carboxylic acid                                   ______________________________________                                    

The salt-containing wellbore fluid of the present invention is preparedby dissolving the salt in the organic fluid, preferably with vigorousstirring. Generally, the salt is added slowly or incrementally to theorganic fluid to allow the added salt to dissolve prior to adding anysignificant amount of additional salt. While heat can be employed toincrease the dissolution rate of the salt in the organic fluid, it ispreferred to not use heat in order to avoid potential detrimentalchemical reactions and/or thermal degradation of the organic fluid. Inaddition, acids, bases, buffering agents, and antioxidants are typicallyadded to the organic fluid either before, during, or after the additionof the salt.

When a hydrocarbon diluent is employed in a wellbore fluid comprising anorganic fluid and a dissolved salt, the dissolved salt-containingorganic fluid and hydrocarbon diluent are combined and any additionaladditive (e.g., hydraulic fracturing proppants, gravel pack particulateagents, viscosifiers, corrosion inhibitors, fluid loss control agents,and organophilic clays) is usually added to resulting combination. Inthose instances where a hydrocarbon diluent is not used, the additionaladditives are preferably added to the dissolved salt-containing organicfluid.

When a salt is not used as a weighting agent, the organic fluid andhydrocarbon diluent are combined and any additional additive (e.g.,hydraulic fracturing proppants, gravel pack particulate agents,viscosifiers, corrosion inhibitors, fluid loss control agents, andorganophilic clays) is usually added to resulting combination. In thoseinstances where a hydrocarbon diluent is not used, the additionaladditives are preferably added to the organic fluid. In addition, whenan aromatic solvent is employed as the wellbore fluid, any additionaladditives are typically added to the aromatic solvent.

The resulting wellbore fluid is preferably stored under conditions whichprevent photochemical reactions (e.g., stored in dark glass or metalcontainers) and oxidation (stored in containers with little, if any, airspace).

The specific techniques used when employing the wellbore fluid of thisinvention are determined by its intended use and are analogous tomethodologies employed when using prior art wellbore fluids forcorresponding well drilling or completion or work-over operations. Forexample, when the wellbore fluid is employed as a gravel packing fluid,it is typically injected into the formation in accordance with theprocedure discussed in U.S. Pat. No. 4,552,215, this patent beingincorporated herein in its entirety by reference.

When employed as a fracturing fluid, the wellbore fluid of the presentinvention is usually injected or pumped into the formation usingprocedures analogous to those disclosed in U.S. Pat. No. 4,488,975, U.S.Pat. No. 4,553,601, Howard et al., Hydraulic Fracturing, Society ofPetroleum Engineers of the American Institute of Mining, Metallurgical,and Petroleum Engineers, Inc., New York, N.Y. (1970), and Allen et al.,Production Operations, Well completions, Workover, and Stimulation, 3rdEdition, volume 2, Oil & Gas Consultants International, Inc., Tulsa,Oklahoma (1989) (Allen), chapter 8, these publications beingincorporated herein in their entirety by reference.

When employed in a perforating operation, the wellbore fluid of thepresent invention is used according to the methodologies disclosed inchapter 7 of Allen, this publications having been incorporated herein inits entirety by reference.

Techniques for using packer fluids and well killing fluids, such asthose discussed in chapter 8 of Allen, are also applicable to thewellbore fluid of the present invention.

EXAMPLES

The following examples are intended to illustrate, and not limit, theinvention. Examples 1-27 demonstrate the dissolution of several salts ina variety of esters and Example 28 details the formation of athree-component wellbore fluid comprising (a) an organic fluid, (b) adissolved salt, and (c) a hydrocarbon diluent. Example 29 sets forth themethodology employed in preparing a two-component system comprising (a)an organic fluid and (b) a hydrocarbon diluent. Core flow tests aredetailed in Examples 30 (using the two-component system prepared inExample 29) and 31 (employing a single-component system, namely, anaromatic solvent).

EXAMPLES 1-27

Dissolution of Salt In Esters

A salt (either CaBr₂, ZnBr₂, or ZnCl₂) was dissolved in each of ninedifferent ester samples to determine the approximate solubility limitsand the viscosities of the resulting fluids. Each salt was weighed outin about 0.5 g increments and then placed in a 50 ml beaker containingabout 5 g of one of the esters. The samples were heated on a hot plateto about 65.6° C. (about 150° F.) to speed up the dissolution of therespective salt. Observations (e.g., rate of dissolution and samplecolor and texture) were recorded. The resulting fluids were cooled toroom temperature and the viscosity of each such fluid was determined atabout 24.4° C. (about 76° F.) in a capillary viscometer. Visualobservation of the rate of dissolution was used to roughly determinewhether a saturated solution was obtained. The results of thisexperiment are shown below in Table A.

                  TABLE A                                                         ______________________________________                                                Vis.sup.1,                                                                            ρ,                                                        Ester Salt                                                                            cp      lb/gal  NS.sup.2                                                                            !.sup.3, g                                                                         Observations                               ______________________________________                                        Isobutyl Isobutyrate                                                          CaBr.sub.2                                                                            N/A.sup.4                                                                             N/A     N/A  N/A   Solidified                                 ZnBr.sub.2                                                                             20     11.1    yes  4     Dissolved Slowly                           ZnCl.sub.2                                                                            280     9.7     yes  3     Dissolved Slowly                           2-Ethoxyethyl Acetate                                                         CaBr.sub.2                                                                            N/A     N/A     N/A  N/A   Reacted Chemically                         ZnBr.sub.2                                                                             300    12.3    no   3.9   Dissolved Quickly                          ZnCl.sub.2                                                                            55      9.8     no   1.8                                              Ethyl Caproate                                                                CaBr.sub.2                                                                            108     9.2     no   2     Turned Cloudy                              ZnBr.sub.2                                                                            108     12.0    no   5     Dissolved Quickly                          ZnCl.sub.2                                                                            20      9.2     no   2.2                                              Ethylhexyl Acetate                                                            CaBr.sub.2                                                                            135     9.2     yes  2     Turned Yellow, CaBr.sub.2                                                     Settled Out                                ZnBr.sub.2                                                                            405     12.1    no   5     Dissolved Quickly                          ZnCl.sub.2                                                                            35      9.1     no   2     Dissolved Slowly                           2-(2-Ethoxyethoxy) ethyl Acetate                                              CaBr.sub.2                                                                            N/A     10.2    yes  1.7   Reacted, Formed Two                                                           Phases                                     ZnBr.sub.2                                                                             428    13.6    no   5     Dissolved Quickly                          ZnCl.sub.2                                                                            >1200   11.4    yes  3.3   Turned Yellow,                                                                Dissolved Slowly                           2-(2-Butoxyethoxy) ethyl Acetate                                              CaBr.sub.2                                                                            26      9.1     yes  0.8   Dissolved Slowly,                                                             Turned Orange                              ZnBr.sub.2                                                                            710     13.3    no   5     Turned Yellowish                           ZnCl.sub.2                                                                            50      9.8     yes  1.7   Dissolved Slowly                           Tributyl Phosphate                                                            CaBr.sub.2                                                                            >1200   10.2    yes  2     Dissolved Quickly,                                                            Turned Yellow                              ZnBr.sub.2                                                                            65      11.9    no   3.5   Dissolved Slowly                           ZnCl.sub.2                                                                            53      10      yes  2     Dissolved Slowly                           Diethyl Phthalate                                                             CaBr.sub.2                                                                            N/A     N/A     N/A  N/A   Solidified                                 ZnBr.sub.2                                                                             >1200  13.4    no   3.6   Dissolved Quickly                          ZnCl.sub.2                                                                            108     10.5    no   1.1                                              Dibutyl Phthalate                                                             CaBr.sub.2                                                                            157     9.9     yes  1     White Opaque Color                         ZnBr.sub.2                                                                            165     10.6    no   1.5   Dissolved Slowly                           ZnCl.sub.2                                                                            165     9.8     no   1     Dissolved Slowly                           ______________________________________                                         .sup.1 "Vis" denotes viscosity.                                               .sup.2 "NS" denotes nearly saturated.                                         .sup.3 " !" denotes the weight of the salt dissolved in the ester.       

The data shown in Table A indicates that various salts can be dissolvedin organic fluids to form a wellbore fluid having a higher density thanthe respective constituent organic fluids.

EXAMPLE 28

Zinc Bromide-Containing n-Octanol in Diesel Diluent

At ambient room temperature (about 20.1° C. (about 70° F.)), anhydrouszinc bromide (about 100 g) was added to about 100 g n-octanol in about20 g increments while stirring the sample vigorously with a stirringbar. After the zinc bromide was completely dissolved, about 250 g of No.2 diesel diluent were added to the zinc bromide-containing n-octanolusing gentle mixing. The resulting wellbore fluid (about 450 g) had adensity of about 8.5 pounds per gallon.

The above Example 28 demonstrates that salts can be dissolved in anorganic fluid and the resulting combination diluted with a hydrocarbondiluent.

EXAMPLE 29

Preparation of Two-Component System

At ambient room temperature (about 21.1° C. (70° F.)), diethyl phthalate(about 315 g) was added to kerosine (about 185 g) gradually in about 30g increments while vigorously stirring the sample with a stirring bar.The resulting wellbore fluid (about 500 g) had a density of about 1.018g/ml (8.5 ppg).

EXAMPLE 30

Core Flow Test Employing a Two-Component System

Core Sample

The core sample employed in this experiment was a water-sensitive WestForeland core having the following approximate characteristics:

    ______________________________________                                        Air permeability  100 md                                                      Porosity          20-22%                                                      Medium Grain Sand                                                             Pore Lining Smectite                                                                            2-3%                                                        ______________________________________                                    

Test Protocol

Kerosine (87 pore volumes) were flowed through the core at ambientconditions using a pressure differential of about 30 psi. The flow ratewas allowed to line out. After the flow rate lined out, thetwo-component system prepared in Example 29 (about 32 pore volumes) wasflowed through the core using a pressure differential of about 30 psi.The flow rate was again allowed to line out. Finally, kerosine was againflowed through the core using a pressure differential of about 30 psiand the flow rate was also allowed to line out to obtain the returnpermeability.

Test Results

The kerosine flow rate prior to passing the two-component system throughthe core was about 3.7 cc/min and after passing the two-component systemthrough the core was about 3.5 cc/min. Hence, the return permeabilitywas about 95%.

EXAMPLE 31

Core Flow Test Employing An Aromatic Solvent

Core Sample

Same as employed in Example 30.

Test Protocol

Kerosine (121 pore volumes) were flowed through the core at ambientconditions using a pressure differential of about 30 psi. PANASOL AN-3Sbrand aromatic solvent (about 82 pore volumes) was flowed through thecore using a pressure differential of about 30 psi. Finally, kerosine(about 47 pore volumes) was again flowed through the core using apressure differential of about 30 psi to obtain the return permeability.

Test Results

The kerosine flow rate prior to passing the PANASOL AN-3S brand aromaticsolvent through the core was about 5.1 cc/min and after passing thePANASOL AN-3S brand aromatic solvent through the core was about 5.0cc/min. Hence, the return permeability was about 98%.

Although the present invention has been described in detail withreference to some preferred versions, other versions are possible. Forexample, in another version of the invention, the organic fluid isemployed as the wellbore fluid in well drilling or completion orwork-over operations, with or without one or more of the optionaladditives (e.g., hydrocarbon diluents, hydraulic fracturing proppants,gravel pack particulate agents, corrosion inhibitors, acids, bases,buffers, viscosifiers, antioxidants, organophilic clays, and fluid losscontrol agents). Therefore, the spirit and scope of the appended claimsshould not necessarily be limited to the description of the preferredversions contained herein.

What is claimed is:
 1. A method for the completion or work-over of awell comprising the step of perforating in the presence of a completionor work-over fluid, the method being characterized in that the fluidcomprises:(a) an alkyl naphthalene-containing aromatic solvent having adensity at about 15.6° C. (60° F.) of at least about 0.9 g/ml (7.5pounds per gallon (ppg)), a flash point of greater than about 54.4° C.(130° F.), a solubility in water at about 25° C. (77° F.) of less thanabout 1 weight percent, a solubility in benzene at about 25° C. (77° F.)of at least about 80 weight percent, a viscosity at about 37.8° C. (100°F.) of less than about 0.2 newton second/meter² (200 cps), and a pourpoint of less than about 15.6° C. (60° F.); and (b) a dialkyl phthalatehaving a solubility in benzene at about 25° C. (77° F.) of at leastabout 80 weight percent, a density at about 15.6° C. (60° F.) of atleast about 1 g/ml (8.35 pounds per gallon (ppg)), and the formula##STR1## wherein R₁ and R₂ are independently selected from the groupconsisting of alkyl groups containing 1 to 4 carbon atoms.
 2. The methodof claim 1 wherein the dialkyl phthalate comprises dimethyl phthalate.3. The method of claim 1 wherein the dialkyl phthalate comprises diethylphthalate.
 4. The method of claim 1 wherein the dialkyl phthalatecomprises dipropyl phthalate.
 5. The method of claim 1 wherein thedialkyl phthalate comprises dibutyl phthalate.
 6. A method for thecompletion or work-over of a well comprising the step of perforating inthe presence of a completion or work-over fluid, the method beingcharacterized in that the completion or work-over fluid is non-aqueousand devoid of solids and comprises a dialkyl phthalate having asolubility in benzene at about 25° C. (77° F.) of at least about 80weight percent, a density at about 15.6° C. (60° F.) of at least about 1g/ml (8.35 pounds per gallon (ppg)), and the formula ##STR2## wherein R₁and R₂ are independently selected from the group consisting of alkylgroups containing 1 to 4 carbon atoms.
 7. The method of claim 6 whereinthe dialkyl phthalate has a melting point less than about 16° C. (60°F.).
 8. The method of claim 6 wherein the dialkyl phthalate has a flashpoint greater than about 60° C. (140° F.).
 9. A method for thecompletion or work-over of a well comprising the step of perforating inthe presence of a completion or work-over fluid, the method beingcharacterized in that the fluid is non-aqueous and comprises an alkylnaphthalene having a density at about 15.6° C. (60° F.) of at leastabout 0.9 g/ml (7.5 pounds per gallon (ppg)), a flash point of greaterthan about 54.4° C. (130° F.), a solubility in water at about 25° C.(77° F.) of less than about 1 weight percent, a solubility in benzene atabout 25° C. (77° F.) of at least about 80 weight percent, a viscosityat about 37.8° C. (100° F.) of less than about 0.2 newton second/meter²(200 cps), an aromatic content of at least about 75 percent, a boilingpoint range which includes, as part of the range, the temperature of247.8° C. (478° F.), and a pour point of less than about 15.6° C. (60°F.).
 10. The method of claim 6 wherein the fluid further comprises ahydrocarbon diluent.
 11. The method of claim 6 wherein the fluid furthercomprises a hydrocarbon diluent selected from the group consisting ofcrude oil, kerosene, diesel oil, polyalphaolefins, mineral oil,gasoline, and naphtha.
 12. The method of claim 1 wherein the aromaticsolvent has an aromatic content of at least about 75 percent.
 13. Themethod of claim 1 wherein the aromatic solvent has a boiling point rangewhich includes, as part of the range, the temperature of 247.8° C. (478°F.).
 14. The method of claim 1 wherein the fluid is non-aqueous.